1. Field of the Invention
The present invention relates to relates generally to the manufacturing of optical fiber components and particularly to a method and apparatus for precisely controlling the optical path length of an optical fiber component.
2. Technical Background
Optical fiber based devices are widely utilized as components for optical communications due to their relatively low insertion loss and low cost. Foremost of optical fiber components are fiber Bragg gratings (FBG) which are typically made by ultraviolet (UV) wavelength energy exposure. Once an FBG is mounted to a substrate and annealed, it is no longer photosensitive and cannot be further tuned. Thus, it is necessary to empirically predict the final frequency of such a grating which can lead to a significant error resulting in gratings which are not within specifications. Due to the uncertainty of the wavelength shift resulting from the attachment process and annealing, the center wavelength (CWL) of a packaged fiber Bragg grating can vary as much as +/-60 picometers from the desired CWL. Such a wavelength error combined with a wavelength drift of, for example, distributed feedback lasers, which may be from +/-50 picometers, and the residual temperature dependence of +/-20 picometers imposes a highly stringent requirement on the design of, for example, 50 GHz fiber Bragg gratings.
A typical attachment process for a fiber Bragg grating is to bond one end of the fiber to a substrate, tension the fiber by an empirically determined amount, and bonding the opposite end of the fiber. FIG. 1 shows the distribution of the CWL for samples manufactured by this process. Since the total available margin is only +/-40 picometers, only a fraction (20% to 30%) of the gratings can be employed.
Precise control of optically tuned fiber-optic devices with a CWL within less than +/-15 picometers is desired to minimize cross-talk between adjacent communication channels of a system. In order to maintain the CWL of a tuned fiber-optic device, such as a fiber Bragg grating, a .beta.-eucryptite substrate has been employed having a coefficient of temperature expansion of -7.5 ppm/.degree. C. to compensate for the refractive index change of the fiber with temperature variations. With such substrates, the CWL shift due to temperature changes over a range of from 0.degree. C. to 70.degree. C. has been reduced to +/-15 pm. Thus, although the substrate selection has improved the stability of the device once manufactured, there remains a need to manufacture devices such as fiber Bragg gratings or other optically tuned components to a CWL that produces a yield rate for precise CWL devices higher than that previously available with existing manufacturing techniques.
It has been discovered that the variability of the CWL of fiber-optic devices is not a function of the laser power employed in the manufacturing of the devices nor is it a result of the substrate material. Instead, it appears that the variability is inherent in the attachment process and there remains a need, therefore, for a process and system for manufacturing precisely tuned fiber-optic devices.